1. Field of the Invention
The present invention relates to a thermostat that performs providing or interrupting electrical continuity in response to temperature changes. More specifically, the present invention relates to a manual-reset thermostat that, once a bimetal performs a reverse action, maintains a reversely-curved state until a reset shaft is pushed.
2. Description of the Related Art
A thermostat is an electronic part that performs providing or interrupting electrical continuity by opening or closing contact between connecting terminals making use of the fact that a disk-shaped bimetal performs a snap action in response to temperature changes. In more detail, electrical continuity is interrupted by opening (i.e., breaking) contact between connecting terminals in response to a reverse action of a bimetal when a predetermined temperature is reached by a temperature rise, whereas electrical continuity is re provided between the connecting terminals by closing (i.e., making) contact between the connecting terminals in response to a return action of the bimetal when another predetermined temperature is reached by a temperature drop.
Herein, if a temperature at which the bimetal is reset is set below a normal temperature, and once the bimetal performs a reverse action, the bimetal maintains a reversely-curved state even if the normal temperature is reached by a temperature drop unless the bimetal is manually allowed to perform a return action. This type of thermostat is called a “manual-reset thermostat.” This manual-reset thermostat is used in various apparatuses such as copying machines. If an abnormality arises in an apparatus in which the manual-reset thermostat is used causes the apparatus to reach a high temperature, the electric current is shut off by allowing the bimetal to perform a reverse action, and the apparatus is stopped. Since this state is maintained even when the normal temperature is reached, causes of the abnormality occurred in the apparatus is removed, and thereafter the bimetal is manually reset by pushing a reset shaft.
FIG. 10A, FIG. 10B, and FIG. 10C are sectional views, each showing a conventional manual-reset thermostat, in which FIG. 10A shows an initial state, FIG. 10B shows a state in which a bimetal “A” has performed a reverse action, and FIG. 10C shows a state in which the bimetal “A” has returned to the initial state by pushing a reset shaft B. This thermostat includes the bimetal “A” that performs a snap operation, a guide pin C disposed under the bimetal “A,” a leaf spring D to which the reverse action of the bimetal “A” is transmitted through the guide pin C, the reset shaft B disposed under the guide pin C with the leaf spring D therebetween, a movable contact E provided at an end of the leaf spring D, and a stationary contact F that is fixed facing the upper part of the movable contact E.
This conventional thermostat is used in the state of FIG. 10A. The bimetal “A” is kept convex upward, and, when the movable contact E and the stationary contact F come into contact with each other, an electric current is applied to an apparatus having this thermostat. However, when a predetermined temperature is reached owing to an abnormality of the apparatus, the bimetal “A” performs a reverse action and reaches a state of being convex downward as shown in FIG. 10B, so that the guide pin C is pushed downward. Accordingly, the leaf spring D is depressed through the guide pin C, and the movable contact E recedes from the stationary contact F downward. As a result, the movable contact E and the stationary contact F are separated from each other, and the electric current to the apparatus is shut off.
In this thermostat, once the bimetal “A” performs a reverse action, the reversely-curved state of the bimetal “A” is maintained even if the temperature falls to a normal temperature. Therefore, in order to return the thermostat to the initial state of FIG. 10A, the reset shaft B is required to be pushed as shown in FIG. 10C. When the reset shaft B is pushed, the bimetal “A” is pushed upward through the leaf spring D and the guide pin C, and the bimetal “A” is returned to the state of being convex upward. When the reset shaft B is brought into a free state after having pushed the reset shaft B, the thermostat returns to the state of FIG. 10A, but the movable contact E and the stationary contact F continue to be in contact with each other by an upward urging force of the leaf spring D.
However, in this conventional thermostat, if the reset shaft B of the thermostat continues to be pushed owing to some circumstances when the apparatus is being used, the state of FIG. 10C is maintained. If so, the bimetal “A” cannot perform a reverse action even when a predetermined temperature is reached later, so that the contacts cannot be separated from each other. Therefore, there has been a fear that the electric current will continue to be applied damaging the apparatus.
From this fact, the present patent applicant has proposed a thermostat disclosed in Japanese Unexamined Patent Application Publication No. H9-198980. FIG. 11A, FIG. 11B, and FIG. 11C are sectional views, each showing this conventional manual-reset thermostat, in which FIG. 11A shows an initial state, FIG. 11B shows a state in which the bimetal “A” has performed a reverse action, and FIG. 11C shows a state in which the bimetal “A” has been returned to the initial state by pushing the reset shaft B.
As shown in FIGS. 11A to 11C, this conventional thermostat includes two make-and-break contacts consisting of movable contacts E1 and E2 and stationary contacts F1 and F2. The movable contacts E1 and E2 are provided at both ends, respectively, of a plate D1. When the bimetal “A” performs a reverse action, one of the two make-and-break contacts is opened, i.e., the contacts E1 and F1 are opened. On the other hand, when the reset shaft B is pushed, the other make-and-break contact is opened, i.e., the contacts E2 and F2 are opened. Therefore, an electric current to an apparatus having this thermostat can be shut off in a state in which the reset shaft B is being pushed.
However, the conventional thermostat of FIGS. 11A to 11C needs the two make-and-break contacts, and, accordingly, the number of constituent elements becomes large, and the structure thereof becomes complex, and costs become high. It is therefore an object of the present invention to provide a manual-reset thermostat that can interrupt electrical continuity in a state in which a reset shaft is being pushed and that has a simple structure.